Grid connected energy storage appears to be gaining interest from utility companies seeking to secure a more stable energy source without the use of expensive peaking plants, as is the current tradition.
Power demand profiles often vary significantly throughout the day, and to meet peak demand, electricity providers often size equipment for peak demand rather than average demand. In turn, higher peak power rates are charged to power consumers as a result of the increase in per unit cost to generate peak demand power.
In light of the foregoing, a benefit of grid connected energy storage may be the ability to better level the demand curve, by using alternative fluctuating energy sources at off peak hours to store energy for use during peak hours. However, as indicated above, commercial implementation of such devices appears to be limited, with peaking plants historically being favored over storage technologies.
Now, in addition to the high cost of peaking plants, more recent concerns over reducing the generation of greenhouse gases, as well as energy shortages (e.g. rolling “brown outs”) particularly in U.S. coastal markets, has also increased interest for the use of a “smart grid”. Such a grid may use a combination of advanced metering, load prediction, and increased electricity production efficiency to satisfy increasing energy demands while minimizing environmental impact.
As part of a smart grid, it may be desirable to use renewable energy sources (i.e. energy from natural resources such as wind, sunlight (solar), rain, tides and geothermal heat), as an alternative to petroleum-based fuels to provide reductions in greenhouse gas emissions. However, many renewable energy sources may not provide the same level of constant power as petroleum-based fuels, and often produce power at off-peak hours when it is not necessarily needed. In other words, the existence of renewable energy sources may fluctuate widely during the day, the existence of which may not be controllable. For example, while electrical power generated from solar radiation may exist during the daylight hours, such solar radiation and corresponding electrical power may not exist at night. Consequently, with greater use of renewable sources, electricity providers may be faced with further increasing the number of installed peaking plants, or use a form of energy storage for power supply smoothing.
There are several advantages that can be realized with increased used of energy storage. The dependence on expensive peaking power plants for peak electricity demand can be reduced. Storage will aid to smooth power production due to fluctuating energy supplies and varying energy demand throughout the day. This will level the demand curve and allow excess energy to be stored during off-peak hours. Energy storage will also allow the majority of plants to be designed and operated at their best efficiency points. The plants will not need extra operation capacity to meet peak demands.
As indicated above, one advantage of energy storage is that may provide a means to enable the use of energy sources with greater fluctuations, such as may be encountered with certain renewable energy sources. Energy storage may be used to bridge the gap between renewable energy production and peak energy demand. Furthermore, to better enable use of a smart grid, energy storage may be used in load leveling, enabling distributed technologies, and increasing the “plant to user” electrical efficiency required to supply the increasing demand on electrical grids.
In order to realize several of the advantages of energy storage listed above (provide excess power during peak demand, store large amounts of energy during off-peak hours from fluctuating energy sources) a large storage capacity is needed. There is a requirement for quick response to electrical grid fluctuations, but in general a long term supply is needed to meet power demand.
One technology that may be used for energy storage is compressed air energy storage (CAES). CAES may be used to store or smooth mechanical or electrical energy from fluctuating power sources, such as wind turbines or solar photo-voltaic panels. Unlike chemical storage, CAES is not understood to utilize expensive (exotic) materials, degrade over time (like batteries), or create an environmental recycling/disposal/landfill problem.
Traditional CAES systems may utilize centrifugal compressors and radial inflow turbines for the compression and expansion processes. Unfortunately, these machines suffer from not having high efficiency, which is undesirable. Furthermore, to avoid large storage tanks for CAES storage (if air is not stored in a geological formation), very high air pressures are desired, which make the usage of centrifugal compressors difficult because multiple stages with individual wheels, bearings, and seals are required to achieve to high pressures with centrifugal compressors.
Alternatively, conventional reciprocating compressors have not been understood to be used in CAES applications given the devices appear to have had some mechanical and pulsation limitations that can result in low reliability and high maintenance costs. These limitations are understood as follows:
Conventional reciprocating compressors and expanders utilize a double-acting piston (inside a cylinder) connected to a rod, crankshaft, coupling, and motor/generator. This arrangement is mechanically complex and inefficient as it consists of multiple moving parts that require bearings, seals, and lubrication.
Conventional reciprocating compressors/expanders utilize mechanical check-valves that are prone to fail under high-cycle fatigue. These valves are understood to be a cause of failures and downtime of reciprocating compressors. Also, in high-speed reciprocating compression, the high velocities of the gas may cause significant valve losses which reduce the efficiency of compression.
Pulsations generated by a conventional reciprocating compressor have to be dampened using bottles, orifice plates, choke tube, and Helmholtz resonators, which add cost, complexity, and efficiency losses to the compressor.
Due to the many moving parts of a conventional reciprocating compressor/expander, speed variation and, therefore, flow capacity control is difficult, as each moving part has its own natural vibration frequency and high-cycle fatigue limitation.
However, the foregoing limitations of conventional reciprocating machinery may be overcome with the new inventions disclosed herein. Furthermore, the new inventions disclosed herein may raise CAES storage-conversion process efficiencies. Thus, a prior limitation of CAES technology, specifically low storage-conversion efficiency, may be resolved using new and advanced technology as described hereinafter.